The method of scheduling mobile stations in a radio communication system is usually performed in such a way, that the performance in terms of data throughput for the radio communication system is optimized.
In an SDMA radio communication system (SDMA=space division multiple access) multiple mobile stations can be scheduled by a base station using a MIMO technique (MIMO=multiple input multiple output) at a same frequency/time resource or same frequency/time/code resource by spatial separation of the mobile stations.
The same frequency/time resource means that two or more mobile stations receive downlink data at same frequency and same time. In this case space is used as an additional multiple access dimension instead of time and frequency to have an orthogonal component in the downlink data for crosstalk avoidance and minimised interference effects.
The same frequency/time/code resource means that two or more mobile stations receive downlink data at same frequency, same time, and same spreading code in case of a CDMA component in the downlink channel. In this case space is used as an additional multiple access dimension instead of time, frequency and code to have an orthogonal component in the downlink data for crosstalk avoidance and minimised interference effects.
In EP 1 784 032 A1 a method for determining a set of mobile stations for an identical basic resource unit in a base station is disclosed. The determination is based on uplink feedback channel information related to a transmit covariance matrix and information (e.g. estimated SINR of the mobile station receiver (SINR=signal-to-interference-plus-noise ratio)) related to a channel quality.
Feedback reporting information is used in different ways depending on radio communication system type:
In GSM or UMTS radio communication systems (GSM=Global System for Mobile Communication; UMTS=Universal Mobile Telecommunication System) for example, feedback reporting is used for adapting a downlink channel from a base station to a mobile station according to several properties of a downlink path related to the downlink channel. Such properties are for instance: distance between the base station and the mobile station, area constitution (e.g. urban, rural) between the base station and the mobile station, spectral use of the downlink path by other transmitters, speed and direction of movement of the mobile station etc. Within the scope of the feedback reporting the mobile station measures beacons or pilots of the downlink channel and reports measurement information (e.g. channel quality indicator, received signal strength indicator) via an uplink channel to the base station. The base station uses the measurement information to adapt parameters (e.g. transmission power, transmission channel) and to improve characteristics (e.g. power consumption, transmission error rate) of the downlink channel.
In SISO and SIMO radio communication systems (SISO=single input single output; SIMO=single input multiple output) using one transmitting antenna, one data signal per time unit is transmitted from the transmitting antenna resulting in a single downlink beam of the downlink channel. In such a case it is sufficient to report the channel quality indicator or the received signal strength indicator and there is no need to identify the downlink beam and to report a beam indicator for the downlink beam within the measurement information,
In MISO or MIMO radio communication systems (MISO=multiple input single output) at least two transmitting antennas and one (MISO) or more (MIMO) receiving antennas are used. Separately encoded data signals can be transmitted per transmitting antenna via two or more downlink beams of the downlink channel on the same time/frequency/code-multiple access resource. In such a case, in addition to spectral noise, interference between the downlink beams occurs.
For SU-MIMO (SU=single user) in LTE Release 8 (3GPP TS 36.213 V8.2.0) single stream codebook based precoding/beamforming will be used and single user MIMO feedback information comprises three parameters: CQI (CQI=channel quality indication), PMI (PMI=precoding matrix indicator), and RI (RI=rank indication). CGI is a parameter indicating a channel quality of a downlink beam with a largest average received modulated carrier power. For LTE Release 8, the CQI parameter is an index for a transport format of the downlink channel. PMI is a parameter indicating a beam index, which corresponds to a precoding vector of the downlink beam for which the channel quality is reported. The beam index belongs to an entry of a codebook, which contains precoding vectors with all allowed Tx antenna weight combinations. RI is a parameter estimated by the mobile station indicating a number of streams for which best data throughput can be expected.
The MIMO technique described in EP 1 784 032 A1 uses multiple downlink beams, which are directed to multiple mobile stations. It is beneficial for performance and overall data throughput in a radio cell or in a sector of the radio cell, if a downlink beam directed to a specific mobile station generates as few interference as possible to downlink beams directed to other mobile stations. Therefore a base station builds a set of adequate mobile stations for a same frequency time resource. This means that the mobile stations of such a set are scheduled at a same time with a same frequency but with spatial separation of downlink beams, so that the downlink beams generate fewest interference between each other.
In US 2007/0249402 A1 a wireless communication system implements beamforming across multiple omni-directional antennas to create beams at different spatial directions. The wireless communication system allocates resources to a communication link using a combination of beam sets and substantially orthogonal resources in order to provide improved coverage without a corresponding increase in interference.
Starting from the disclosures of US 2007/0249402 A1, for building such a set of adequate mobile stations an appropriate feedback information has to be reported from the mobile stations to the base station, which should not consume too much resources of the uplink channel.